Information
If the pwhistory enforce_for_root option is enabled, the module will enforce password history for the root user as well
By default the password-history check does not apply to the root account, so root can be changed back to a recently used password even when history enforcement is configured for other users. Enabling enforce_for_root subjects root's password changes to the same history check. Because the root account is a high-value target, preventing reuse of a recent - and possibly compromised - root password closes that gap.
Solution
Edit or add the following line in /etc/security/pwhistory.conf :
enforce_for_root